The present invention relates generally to methods and apparatuses for manufacturing the molded articles, and more particularly to a method for modifying a surface of thermoplastic resin (or molten resin) using the supercritical fluid, and an injection molding method using the method.
Various processes have recently been proposed which apply the supercritical fluid to resin molding processing. The supercritical fluid is a unique medium that has a solvent characteristic close to the liquid and has such permeability as the gas. For example, one proposed method improves the flow properties and transfer performance of the resin at the time of injection molding by using the supercritical fluid and compressed CO2 that permeate into the thermoplastic resin, serve as a plasticizer, and lower the viscosity of the resin (see, for example, Japanese Patent Application No. 10-128783). The method disclosed in Japanese Patent Application No. 10-128783 previously fills the inert gas at a high-pressure state, such as CO2, in a mold cavity, and then fills the molten resin in the cavity through injection. As a result, the high-pressure gas permeates in the surface from the flow front part of the resin at the time of flowing due to the fountain effect. This method is referred to as a counter pressure method, which is a technique that restrains a growth of a skin layer on the mold surface as an obstacle of the flow and transfer. The pressurized inert gas is used as a counter pressure.
Another proposed method opens, after the resin is filled in a mold cavity, a mold to form an aperture between a mold transfer surface and the resin, injects the CO2 gas, such as supercritical fluid into the aperture, and softens the resin surface. See, for example, Japanese Patent No. 3,445,778. The method disclosed in Japanese Patent No. 3,445,778 is different from the counter pressure method and referred to as a core back method for opening a mold core that restricts the mold cavity after the filling and for injecting CO2.
A still another proposed surface modification molding method develops the above method by utilizing the solvent characteristic of the supercritical fluid (see, for example, Japanese Patent Application, Publication No. 2003-320556). The method disclosed in Japanese Patent Application, Publication No. 2003-320556 uses the compressed gas, such as supercritical CO2, which has solubility to the molten resin to be injected. The method injects into the mold cavity the compressed gas as a counter pressure in which a modifier dissolves or disperses, then injects the molten resin into the mold cavity, and obtains the molded article having a modifier modified surface.
However, the earnest researches by the instant inventors have discovered that the above surface modification injection molding method that uses the supercritical fluid has the following various problems and is hard to commercialize: The supercritical CO2 has a solvent characteristic similar to that of n-hexane and the solvent's solubility too low for the injection molding process even if the entrainer (or assistant) is used. In other words, the concentration of the functional agent as solute is much lower than the concentration of the compressed gas, such as CO2, in the resin. The functional agent permeates only by a small amount from the surface in a short-time contact between the supercritical fluid and the resin as in the counter pressure method and the core back method. While the entire resin surface which the compressed gas contacts becomes soft, it is found that the functional agent does not exhibit an intended function. In other words, the methods in the above prior art references that touches the supercritical fluid and the compressed inert gas to the resin in the mold cannot control the contact time period between the resin and the supercritical fluid and the compressed inert gas, and cannot sufficiently modify the surface.
In addition, the methods in the above prior art references introduce the supercritical fluid that has a certain pressure condition outside the mold and dissolves the solute at that pressure at the saturated solubility, into the mold cavity at the ambient pressure and touches the solute to the resin, inevitably causing the rapid decompression. There is a strong correlation between the supercritical fluid's pressure and the solute's solubility. The rapid decompression remarkably lowers the solubility and the solute separates out. While the pressure returns quickly, the solute once separating out does not immediately dissolve again, causing clogs in a channel in the mold. This problem also makes it difficult to recover and recycle the functional agent. The rapid decompression phenomenon when the supercritical fluid is introduced into the mold is conspicuous in the surface modification molding that needs dissolve the solute, although the phenomenon merely causes the volume expansion with respect to the inert gas, such as CO2, itself.
When the functional material uses the organic metallic complex and is applied to the electroless plating processing, a method that uses the heat etc. to precipitate fine metallic particles is inefficient. In addition, the functional material is unlikely to serve as catalyst cores of the fine metallic particles that have permeated in the resin.